Numerous devices have been suggested for use in administering medical fluids to patients. In particular, there have been numerous attempts to design a reliable, accurate intravenous flow rate indicator/controller that is inexpensive enough to be disposable.
Desirably, such a device should maintain the initially set rate of delivery even if the temperature changes and handle a wide range of fluids without being affected by the differences in viscosity. The device should be easy to prime, handle a wide range of flows, reset automatically to the desired primary flow rate after any secondary fluid delivery ends, and stop fluid delivery before air enters the tube leading from the device to the patient.
If a device can do those things, chances for a medical emergency caused by excessive or insufficient flow of medication or by an air embolism from the procedure are reduced. Additionally, demands on nursing time are lessened because frequent readjustment to the initially set flow rate is not needed and hospital costs are lowered since a hospital need keep only one type of device in stock to handle both pediatric and adult patients (the typical flow rates are significantly different) and all medical fluids normally dispensed (viscosities vary significantly).
The classic intravenous metering set comprises a fluid supply container (commonly of 1 liter volume), a drip chamber, tubing from the bottom of the drip chamber to a needle in the patient, and a device to pinch the tubing to control the flow. Fluid from the supply container drips into the chamber through a standard size cannula at a rate determined by the internal cannula fluid pressure and the gas-phase pressure in the chamber (controlled indirectly by pinching the tubing).
The cannulae are supposed to deliver a fixed number of drops per milliliter (typically 15, 20, or 60), and flow rate is set by pinching the tube to achieve a number of drops per unit time equivalent to the prescribed number of milliliters of medication per hour. However, cold-flow of the tubing material where pinched, which enlarges the cross-sectional flow path, and, in some systems, lowering of the fluid level in the supply container as fluid administration progresses may cause the flow rate to vary significantly from that initially set. Additionally, the flow rate usually will change with variations in the patient's venous pressure and vertical movement of the neddle's point of entry .
Numerous attempts have been made to provide metering devices that are calibrated to indicate the flow rate and are free from these problems. For example, U.S. Pat. Nos. 2,479,786, 3,340,871, 3,938,539, and 4,136,692 disclose devices for metering medical fluids where the flow rate is indicated by the height of the fluid in a chamber. The devices of the first three patents utilize apparatus having apertures in relatively thick walls through which the fluid passes. The fourth, U.S. Pat. No. 4,136,692, makes use of one or more thin-plate orifices, which it discloses are relatively insensitive to variations in fluid viscosity. Additionally, U.S. Pat. No. 3,690,318 discloses an intravenous administration device utilizing a plurality of orifices.
U.S. Pat. Nos. 3,756,233 and 3,931,818 disclose two-chamber intravenous metering devices in which the fluid flow rate is controlled by varying the relative height of the chambers. Additionally, U.S. Pat. No. 3,756,233 discloses venting the device to the atmosphere. The device of U.S. Pat. No. 3,931,818 makes use of tubing having a standard pressure drop to fluidly connect the two chambers.
U.S. Pat. Nos. 3,207,372, 3,227,173, and 3,963,024 disclose devices having float check valves to prevent air from entering the tubing leading from the device to the patient when the fluid in each device is exhausted. U.S. Pat. Nos. 3,967,620 and 4,056,100 discloses the use of hydrophilic material acting as membrane valves for the same purpose. Additionally, the devices of U.S. Pat. No. 3,963,024 utilize two fluid chambers that are gas-phase pressure-equalized.
However, none of these patents provides a reliable, accurate, low-cost intravenous fluid flow rate indicator/controller that is insensitive to ambient temperature, can maintain the initially set flow rate, is easy to prime, resets to the primary flow rate after any secondary fluid administration ends, and indicates flow rate over a wide range of flows, with particular sensitivity at very low flow rates and without regard to fuid viscosity.